- Rename NumbersKeyboardComponent to MathKeyboardComponent
- Decrease the number of input properties
- Split source code into private methods

* Add IntersectionService
* Add CompoundElementComponent as parent for LikertComponent
* Inject ElementRef to common ElementComponent
* Rename players' ElementComponent to ElementContainerComponent

* Remove general (full) keyboard in layout component
* Implement French keyboard component

to avoid name collision.

* Implement as draggable overlay
* Keyboard can be configured as 'mini' or 'full'

* After deleting validation component, element component is no
longer needed and is deleted
* Element overlay takes over the methods of element component

* Delete validation message component
* Use instead `mat-error` inside element components to take advantage
 of `mat-input-field`
* Add get validators method to each form element component
* Display validation messages via `error-transform` pipe

* Bugfix for units without always visible page
* Bugfix for aspect ratio over 50%
* Integration of keyboard

* Move two different intersection detections from
`scroll-index.directive` and `section.component` to
`intersection-detection.directive`

* Fix bug for small pages
* Add functionality for tabbed pages
* Use layoutContainer as container for intersection detection
* Rename properties and methods

Move `MatDialogModule` from editor to common
Add `AlertDialog` to player

When elements are dynamically positioned there is no space below the elements

* Pages can be scrolled or tabbed
* Labels of pages can be shown and hidden
* Always visible pages can be left, right, top or bottom
* Layout of pages is now handled in `LayoutComponent` and removed from
`PlayerStateComponent`
* Due to the `ScrollIndexDirective`, pages with scrollbars have similar
interfaces as pages with tabs

Refactor some translations

more information about [ngx-translate](https://github.com/ngx-translate/core)

In addition to the scrollable pages, units can also have one page that
is always visible.

`PlayerStateComponent` encapsulates information about the status of the
player.

Move specific form functionality from `AppComponent` to a new
`FormComponent` to make the code cleaner

* When creating components for each element a
`ValidationMessageComponent` is also generated dynamically
* Prepare usage of default values for form elements
* Add `form.ts` for form specific interfaces
* Rename some variables and properties

Move all player components to component folder but `AppComponent`
Improve naming of variables

- Replace in the projects 'editor' and 'player' the selector
'app-root' with 'editor-aspect' or 'player-aspect' and make a
case distinction unnecessary
- Remove definitions of the property 'local' in `environment`
and `angular.json`

player and editor can be served without commenting in
app module and app component. Use `start-editor-local` and
`start-player-local`